1. Field of the Invention
The invention relates to polymerizable phthalonitrile compositions, and in particular to storable mixtures of phthalonitrile monomers and curing agents.
2 Description of the Related Art
Phthalonitrile polymers of various types are described generally in U.S. Pat. No. 3,730,946, U.S. Pat. No. 3,763,210, U.S. Pat. No. 3,787,475, U.S. Pat. No. 3,869,499, U.S. Pat. No. 3,972,902, U.S. Pat. No. 4,209,458, U.S. Pat. No. 4,223,123, U.S. Pat. No. 4,226,801, U.S. Pat. No. 4,234,712, U.S. Pat. No. 4,238,601, U.S. Pat. No. 4,259,471, U.S. Pat. No. 4,304,896, U.S. Pat. No. 4,307,035, U.S. Pat. No. 4,315,093, U.S. Pat. No. 4,351,776, U.S. Pat. No. 4,408,035, U.S. Pat. No. 4,409,382, U.S. Pat. No. 4,410,676, U.S. Pat. No. 5,003,039, U.S. Pat. No. 5,003,078, U.S. Pat. No. 5,004,801, U.S. Pat. No. 5,132,396, U.S. Pat. No. 5,159,054, U.S. Pat. No. 5,202,414, U.S. Pat. No. 5,208,318, U.S. Pat. No. 5,237,045, U.S. Pat. No. 5,242,755, U.S. Pat. No. 5,247,060, U.S. Pat. No. 5,292,854, U.S. Pat. No. 5,304,625, U.S. Pat. No. 5,350,828, U.S. Pat. No. 5,352,760, U.S. Pat. No. 5,389,441, U.S. Pat. No. 5,464,926, U.S. patent application by Satya B. Sastri and Teddy M. Keller for "FIBER-REINFORCED PHTHALONITRILE COMPOSITE CURED WITH LOW-REACTIVITY AROMATIC AMINE CURING AGENT" filed Oct. 2, 1997 and U.S. patent application by Satya B. Sastri and Teddy M. Keller for "PHTHALONITRILE THERMOSET POLYMERS AND COMPOSITES CURED WITH HALOGEN-CONTAINING AROMATIC AMINE CURING AGENTS" filed Oct. 2, 1997. All of these patents and applications are incorporated herein by reference.
The above references teach an in situ method of polymerization wherein a phthalonitrile monomer is heated to a temperature above the melting point of the monomer and a curing agent is added to the melt stage to promote polymerization of the phthalonitrile monomer. A method wherein the curing agent is added immediately before the heating step has also been described. These methods of polymerization can be inconvenient in some circumstances, particularly in the fabrication of composites by methods such as resin transfer molding (RTM), filament winding, injection molding, or prepreg formulation, where it may be difficult or cumbersome to mix the monomer and the curing agent on site or to add the curing agent to the melt after the heating step is begun. Alternatively, many of the above references teach that a frangible B-stage polymer can be created by heating the phthalonitrile monomer with a curing agent to initiate the polymerization process and then quenching the reaction before the polymerization goes to completion. The B-stage polymer created by this process can be stored indefinitely at room temperature. However, creation of a B-stage prepolymer involves the additional time-consuming and cost-consuming step of partial polymerization of the monomer.
In conventional resin systems such as epoxies, polyimides, vinyl esters, cyanate esters, etc., a mixture of a monomer and a curing agent must be used fairly soon after the mixture is created because the monomer begins to immediately polymerize in the presence of the curing agent. Even under freezer conditions, the mixture has a short shelf life. As the material polymerizes, it becomes highly viscous and unprocessable. Thus, long term storage of a mixture of the monomer and a curing agent in these conventional resin systems is not possible.